Vapor phase nitration of cyclohexane



United sta e P e iO VAl OR PHASE NITRATION F CYCLOHEXANE John H. Bonfield, Hamburg, N.Y., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Feb. 16,1961, Ser. No. 89,613

i 9 Claims. (Cl. 260-4544) ;This inventionrelates to the production of mono-' nitrated cyclohexane, and more particularly to a continuous vapor phase ,method for producing mononitrated cyclohexane, an important intermediate in the production ,most important, take place and may become predominant,

at the expense of the desired nitration reaction. .In order to control such undesired lside reactions, it has been suggesteld to limit the reaction contact time to the order of ..one or two seconds or less and employ large molar-ratios of 'alkane to nitrating agent. The short contact times and the excess amounts of alkane employed in such procedures, while reducing the danger of explosions, have necessitated uneconomical procedures for separating and freeycling the relatively large proportions ofunreacted Z alkane. I

f Furthermore, because of the-highly exothermic reacfions involved, many of thenitration procedures heretofore suggested present an ever present danger of explosions taking place; I

It is an object of the presentinvention to provide "a safe and efficient process for the mononi-tration of cyclohexane. K A further object of the invention is to provide suchprocess involving high mates of conversion and high yields of the mononitrated cyclohexane. Other objects and advantages of the process of the invention will "be apparent from the following detailed description thereof.

According to the presentinvention, cyclohexane vapors and a nitrating agent comprising nitric acid or nitrogen dioxide (N0 or N 0 01 a mixtureof both). also, in the .Ivapor phase are mixed at a temperature below that at which substantial nitration occurs, i.e., .b eIoWTZSDFlfC in the molar ratio of from 4 upto '10 mols; preferably i from 4 to 5 mols, of cyclohexane .per mol of nitric acid (as 100% l-lNO or-from 1 up to'5 mols, preferably from 1 to 1.5 mols of cyclohexaneipermol of nitrogen dioxide ,(calculated as N0 and this gaseous mixture is passed I through a reaction zone maintained at a temperature between about 25 0 C. and'375 9 C. for a residence time of iat ;least 10.seconds, preferably from 20 seconds to 150 seconds. I c

' Preferably, the temperature and residence times within their specified ranges bear an inverse relation to each other, i.e'., when the temperature is in the upper portion fpttherange 250?. to 375 C. shorter residence times are 3,133,123 Patented May 12, 1964 when the temperature is in thelower portion of the 250 to 375 C. range longer residence times within the range of from 20 to 150 seconds are employed. The optimum residence time and temperature, Within the aforesaid ranges, depend on the reactorin which the. nitration is carried out, the nitnating agent used, whether nitric acid or nitrogen dioxide; and the particular molar propor tions of the reactantsemployed Within the ranges hereinabove disclosed. Preferably, the reactionis carried out kinder substantially atmospheric .pressure lconditions [for the residence time and at the temperature specified. The reaction mixture is removed from the reactionzone and the mononi-trated cyclohexane separated therefrom. The unreacted cyclohexane is recovered from the reaction gases and can be recycled to the nitration reaction zone and eventually substantially entirely converted to mononitrocyclohexane i I conversion ratesof the cyclohexane to mononitrocyclohexane take'place per pass through the react-ion zone and high yields of mononitrocyclohexane are pro:

' duced at'the relatively low reaction temperatures and used the range of frorn20 to 1 50 seconds and oxide, also in the vapor hexane and nin'ating agent.

I plosive.

high residence times employed. Moreover, the reaction proceeds smoothly and safely. Under these conditions of temperature, residence time and reactant proportions (the combination of all three is important), no danger ofexplosionarises. I I

According to a further feature of the present invention, at least 0.5 mol of nitric oxide (NO) per mol of nitrating agent is incorporated in the gaseous reactionmixtu're prior to the nitration reaction. Such addition of nitric oxide unexpectedly increases the yield of nitrated cyclo hexane.

The cyclohexane in the vapor phase is admixed with the nitrating agent comprising nitric acid or nitrogen diphase, at a temperature below that at which substantial nitnation of the hydrocarbon occurs, i.e., below about 250 C., and preferably between about C. and 180 C. This pre-mixing step avoids the production of localized explosive mixture of cyclo- -It is important to mix the cyclohex-ane vapors with vapors of the initiating agent because liquid solutions ofthe cyclohexane in the nitrating agent, particularly nitrogen dioxide (in they form of nitrogen tetroxide), are :known to be unpredictably ex- When nitricacid is employed as the nitrating agent, at least4 mols and preferably 5 mols, but less than 1() mols, of the cyclohexane are mixed Nitric "acid of commencemay" beused (concentration about 68%) or nitric acid of any available concentration, the more concentrated the better. When nitrogen dioxide is employed, at least. an equimolar ratio of reactants should be used, and it is desirable to utilize a ratio of 3 the cyclohexane per mol'of nitrogen. about 114 mols, but less than 5 mols of lcyclohexane per mol of nitrogen dioxide.

about 1.5 mols of dioxide, preferably It is preferred to employnitro'gen dioxide rather than nitric acid as the nitrating' agent because the former materialcan be utilized with lesser molar excesses of the cyclohexane, thereby reducing the proportion of unreacted cyclohexane recycled to the reaction zon e. Whether nitric acid or nitrogen dioxide is used as the nitrating agent, utilization of the above specified molar ratios of with each mol of nitric acid. 1

cyclohexane to nitrating agent, the temperature conditions and residence times above disclosed insure safety from explosion hazards.

The mixture of cyclohexane vapors and gaseous nitrating agent is passed through a reaction zone maintained at a temperature between 250 C. and 375 C. for a residence time of at least 20 seconds. Since reactor dimensions must be increased with increasing residence time, and since the quality of the nitrated product decreases as residence times are increased above 70 seconds, it is preferred to employ residence times of from 20 to 70 seconds; residence times of from 40 to 50 seconds are especially preferred, among other reasons, because the yield of mononitrated cyclohexane formed in the reaction zone reaches optimum values at residence times within this range. Using nitric acid as the nitrating agent, the preferred reaction temperature is between 290" and 310 C. Employing nitrogen dioxide, the preferred temperature of operation is between 265 and 285 C.

As noted, nitric oxide may be admixed with the reaction mixture either by addition to the nitrating agent or to the mixture of reactants. The addition of nitric oxide increases the yield of the mononitrated cyclohexane. While the explanation for this phenomenon is not fully understood, it is believed that the nitric oxide serves to minimize the concentration of oxygen in the reaction mixture. It is known that nitric acid dissociates completely at temperatures above about 250 C. according to the reaction:

Additionally, nitrogen dioxide dissociates above about 150 C. with increasing temperature to produce nitric oxide and oxygen by the reaction:

It is believed that the introduction of nitric oxide into the reaction mixture, especially when added prior to exposure to reaction temperatures, i.e., prior to raising the temperature of the reaction mixture to between about 250 C. and 375 C., serves to suppress the dissociation Reaction II and thus to minimize the concentration of oxygen in the reaction mixture.

' The nitric oxide is preferably added to the reaction mixture during the pre-mixing step; alternatively, the nitric oxide may be added at any time prior to that at which the nitrogen dioxide undergoes any appreciable dissociation. The amount of nitric oxide admixed with the reactants is preferably from 0.5 to 1 mol of nitric oxide per mol of nitrating agent. Greater proportions of nitric oxide than 1 mol per mol of nitrating agent can be used though such additional amounts are not significantly more efiective in suppressing the dissociation. Further, the nitric oxide acts as an inert diluent and thus shortens the effective contact time between the reactants and tends to reduce the yield. Accordingly, excesses of nitric oxide above 1 mol per mol of nitrating agent should be avoided. 1

While the admixture of nitric oxide with the reactants is preferred, this invention is not limited to such procedures but includes nitrations of cyclohexane under the specified conditions without the addition of nitric oxide to the reactants.

The condensable portion of reaction mixture from the reaction zone is condensed, the unreacted cyclohexane separated, for example, by fractional distillation, from the reaction mixture and, if desired, recycled to the reaction zone. From the residue, nitrocyclohexane can be recovered in any suitable manner, as by fractional distillation, conversion to the ammonium salt, etc. The noncondensable portion, comprising oxides of nitrogen can be oxidized, e.g., with hydrogen peroxide, to form nitric acid.

The reaction is carried out by flowing separate streams of the cyclohexane vapor and the nitrating agent, also in the vapor phase, through preheaters, then mixing the preheated vapors to form a vapor mixture at a temperature below 250 C. and passing this mixture through the reaction zone maintained at the reaction temperature. Al ternatively, cyclohexane vapors may be mixed with vapors of the nitrating agent, the resultant vapor mixture preheated to a temperature below the reaction temperature and the preheated mixture introduced into the reaction zone. The mixture is passed continuously through the reaction zone which is so dimensioned as to give the desired residence time.

The equipment used for the practice of the process should, of course, be resistant to corrosion by nitric acid; glass, glass-lined or acid-resistant stainless steels can be used.

The following examples in which temperatures are given in C., and parts and percentages are on a weight basis unless otherwise specified, illustrate preferred modes of practicing the invention. It will be understood the invention is not limited to these examples.

EXAMPLE 1 cyclohexane and 68% nitric acid were vaporized separately and the vapors mixed at about 180 at the hourly rate of 271 parts of cyclohexane and 60 parts (40.8 parts HNO of nitric acid. The mixed vapors were then passed into a spherical glass reactor of 550 parts volume held at 350 by a molten alloy bath. The residence time of the mixed vapors within the reactor was about 35 seconds. The gases emanating from the reaction vessel were condensed and the non-condensable portion was bubbled into a solution of hydrogen peroxide.

The liquid condensed from the reaction gases was permitted to stratify intotwo layers, an organic layer containing unreacted cyclohexane and the nitrocyclohexane and an aqueous layer containing 7.2 parts of nitric acid. The organic layer was distilled in vacuo to separate the unreacted cyclohexane. The residue was then distilled with steam to yield a light yellow oil, which on fractional distillation yielded 11.8 parts of pure nitrocyclohexane (B.P. 76/10 mm. Hg abs. pressure).

The uncondensed gases were passed into the hydrogen peroxide and yielded 20.65 parts of nitric acid. At the end of one hour, 009 mol of nitrocyclohexane was produced; the conversion was 14.1% and the yield was 43.5% based on nitric acid consumed.

EXAMPLES 2 TO 6 Examples 2 to 6, inclusive, were carried out using the same procedure as described in Example 1, except for the residence times and reaction temperatures noted in Table I below. The molar ratio of cyclohexane to nitric acid was 4 to 1. This series of examples illustrates that the residence time and reaction temperature within their respective specified ranges bear an inverse relation to each other.

Table I Yield (M01 Residence Percent) Example Time, Sec. Temp. Based on Gyclohexane Consumed EXAMPLE 7 In a two-hour period, 538 parts of cyclohexane and 209.2 parts of nitrogen dioxide (from nitrogen tetroxide) were vaporized and mixed at about The mixture was heated in a spherical reactor to 275 and maintained therein for a period of 42.5 seconds. Thereafter, the reacted mixture was passed through a condenser and the condensables separated from the reaction off-gas which was essentially nitric oxide.

The condensed reaction products were permitted to stratify. The organic layer weighing 608 parts was sepa in Example 7.

The organic layer was representsa yield of 63% based on cyclohexane conf sumed, and a conversion of 22.3%.

"From the oif-gasan amount of nitric acid equiv-alent to 109 parts of nitrogen dioxide'was obtained; Thus the yield with respect to nitrogen. dioxide consumed was s 40.6% and the conversion rate was 48.0%. p

. EXAMPLES 8 r0 .12,INCLUSIVE i In a similar manner to that described in above Example 7, cyclohexane and nitrogen dioxide were reacted for the residence times and at the reaction temperatures indicated in connection with each of Examples 8 to 12, in-

clusive, in Table 11 below. -In each instance a ratio of 1.0 mol of N0 to 1.4 mols 'of cyclohexane'was used.

The yield with respect to cyclohexane consumed and the amount of-nitrocyclohexane in the product as produced in these examples are given in Table II as follows:

From this data, it can be seen that the yield and amount of nitrocyclohexane produced increases with the V residence time to a maximum at about 50 seconds and then drops off. It is also evident that the use of nitrogen dioxide as the nitrating agent makes possible operation at lower molar ratios and hence more efficiently and with less recycling.

A further advantageof the present invention is that it results in an improvement in the quality of the crude monoitrocyclohexane. The quality or the purity of the crude nitrocyclohexa-ne obtained after removal of the unreacted cyclohexane is one indication of the efficiency of the nitration. Examples 13 and 14 of this invention were carried out under the conditions of residence time and temperature indicated in Table III below. The reactant proportions in these examples were the same as Table III It will be noted that Examples 13 and 14 gave a crude product containing 74% and 81%, respectively, of mononitra-ted cyclohexane. In the case ofcomparative runs with residence times of less than seconds under optimum temperature conditions, the quality of the crude was markedly inferior to that obtained in the above examples and the yield was significantly lower.

EXAMPLE 15 538 parts of cyclohexane, 209.2 parts of nitrogen di-' oxide and. 13 6 parts of nitric oxide were pre-mixed at a temperature of about 200 and the mixture thus formed was continuously passed through a reactor maintained at 280 at a rate such that the residence time therein was arated from..-the aqueous lawn-consisting of29 parts of V -water formed in the reaction.

' distilled to give 418 parts of unreacted cyclohexane and 141 parts'of crude nitrocyclohexane containing 114 parts of pure nitrocyclohexane. This amount of pure product 62.5'secondsQ The reaction'products were passed through a condenser to remove therefrom condensables and the oft-gas was vented to the atmosphere.

The condensate separated intotwo layers. From the upper organic layer, 425 parts of cyclohexane were'rea covered by distillation. The residueweighed 156.3 parts and contained nitrocyclohexane. Thus, parts of nitrocyclohexane were produced which represented a yield of 72%, based on cyclohexane consumed.

:It will be noted that the present invention provides an efficient and safe process for the vapor phase mononitration of cyclohexane. Since certain changes can be made in oarryihg'out this process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. Moreover, while certain theoretical explanations have been given in the specification to facilitate a better understanding of this invention, it will be understood that this invention is not to be limited in any way by these theoretical explanations.

What is claimed is: r p

1. A process for mononitrating cyclohexane in the vapor phase comprising, mixing vapors of cyclohexane and vapors of a nitrating agent selected from the group consisting of nitric acid and nitrogen dioxide at a temperature below that at which substantial nitration occurs in the proportions of from 4 up to 10 mols of cyclohexane per mol ofnitric acid when using nitric acid as the nitrating agent and in the proportions of from 1 up to 5 mols of cyclohexane per mol of nitrogen dioxide, calculated as N0 when using'nitrogen dioxide, the said mixing being effected by mixing the vapors of cyclohexaneand the vapors of the nitrating agent with both in the vapor phase and in the absence of liquid phasereactants, passing the resultant gaseous mixture through a reaction zone maintained at a temperature between 250 C. and 375 C. for a residence time of at least 20 seconds, removing the resultant reaction mixture from said reactionzone, and separating the mononitrated cyclohexane from said mixture.

2. The process as defined in claim '1, in which nitric oxide is incorporated in the reaction mixture prior to its passage through said reaction zone.

3. The process as defined in claim 1, in which the gaseous mixture is maintained in the reaction zone for a residence time of from 20 to seconds and the temperature in said reaction zone and said residence time are in inverse relation to each other'within their respective ranges. Y

4. A process for mononitrating cyclohexane in the vapor phase comprising, continuously flowing separate streams of cyclohexane vapor and nitric acid vapor, mixing these two streams to produce a mixture at a temperature between 120 and C. containing from 4 to 5 mols of cyclohexane per mol of nitric acid, continuously streams of cyclohexane vapor and nitrogen dioxide in the gaseous phase, mixing the two streams to produce a gaseous mixture at a temperature within the range of from 120 to 180 C. containing from 1 to 1.5 mols of cyclohexane per mol of nitrogen dioxide, continuously passing said gaseous mixture through a reaction zone maintained at between 250 C. and 375 C. under sub-' stantially atmospheric pressure for a residence time of from 20 to 70 seconds, continuously removing said mix- 7 ture from said zone, and separating mononitrated cyclohexane from said mixture.

7. The process defined in claim 6, in which from 0.5 to 1 mol of nitric oxide per mol of nitrogen dioxide is added to said mixture prior to passage thereof through said reaction zone.

8. A process for mononitrating cyclohexane in the vapor phase comprising, continuously preheating a stream of cyclohexane vapor, separately preheating. a stream of nitrogen dioxide gas, mixing the two preheated streams to produce a gaseous mixture at a temperature between 120 and 180 C. containing from 1 to 1.5 mols of cyclohexane per mol of nitrogen dioxide, continuously reacting said gaseous mixture for a period of from 40 to 50 seconds in a reaction zone maintained at between References Cited in the fiie of this patent UNITED STATES PATENTS 1,967,667 Hass et a1. July 24, 1934 2,071,122 Hass et a1; Feb. 16, 1937 2,512,587 Stengel June 20, 1950 2,894,041 Berg July 7, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CURRECTIQN Patent No. 3,, 133 128 May 12 1964 John Ho Bonfield It is hereby. certified that error appears in the above numbered. patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1 line 58 for "10 read 20 'column 5 line 44 for "monoitrooyolohexane' read mononitrocyclohexane column 5 Table Ill heading to the third column thereof for "degre" read degree Signed and sealed this 22nd day of September 1964.,

( SEAL) Attest:

ERNEST W SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CGRRECTION Patent No. 8 133 123 May 12 1964 John H Bonfield It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected belo* Column l line 58 for "10" read 20 column 5 line 414 for 'monoitrocyclohexane" read mononitrocyclohexane column 5 Table llI heading to the third column thereof for "'degre" read degree Signed and sealed this 22nd day of September 19640 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR MONONITRATING CYCLOHEXANE IN THE VAPOR PHASE COMPRISING, MIXING VAPORS OF CYCLOHEXANE AND VAPORS OF A NITRATING AGENT SELECTED FROM THE GROUP CONSISTING OF NITRIC ACID AND NITROGEN DIOXIDE AT A TEMPERATURE BELOW THAT AT WHICH SUBSTANTIAL NITRATION OCCURS IN THE PROPORTIONS OF FROM 4 UP TO 10 MOLS OF CYCLOHEXANE PER MOL OF NITRIC ACID WHEN USING NITRIC ACID AS THE NITRATING AGENT AND IN THE PROPORTIONS OF FROM 1 UP TO 5 MOLS OF CYCLOHEXANE PER MOL OF NITROGEN DIOXIDE, CALULATED AS NO2, WHEN USING NITROGEN DIOXIDE, THE SAID MIXING BEING EFFECTED BY MIXING THE VAPORS OF CYCLOHEXANE AND THE VAPORS OF THE NITRATING AGENT WITH BOTH IN THE VAPOR PHASE AND IN THE ABSENCE OF LIQUID PHASE REACTANTS, PASSING THE RESULTANT GASEOUS MIXTURE THROUGH A REACTION ZONE MAINTAINED AT A TEMPERATURE BETWEEN 250*C. AND 375* C. FOR A RESIDENCE TIME OF AT LEAST 20 SECONDS, REMOVING THE RESULTANT REACTION MIXTURE FROM SAID REACTION ZONE, AND SEPARATING THE MONONITRATED CYCLOHEXANE FROM SAID MIXTURE. 